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A geologist in the Arctic.
Gneiss, tuff
and a little wacke

Icon geology.png
Know your schist
Rock stars

The science of geology ranges from the study of individual rocks, to the study of the composition of the entire Earth. In between, geologists work to explain why the surface of the land looks the way it does, where valuable mineral resources might be found, how the land used to look based on the minerals deposited there, and how the continents have moved over time, creating new mountain ranges and oceans. Much to the dismay of Fundamentalist Christians, all of this provides evidence for the fact that this is a very old planet. Indeed it would be very difficult to conduct any sort of mineral exploration using a young-Earth hypothesis.

Branches of geology[edit]

  • Geochemistry — Using chemistry to solve problems in geology
  • Geophysics — Using physics to solve problems in geology
  • Bio-geology — Using geology to solve problems in biology
  • Paleontology — Study of fossils, also related to biology
  • Volcanology — Geological study of volcanoes
  • Seismology — Geological study of earthquakes
  • Mineralogy — Geological study of minerals
  • Petrology — Geological study of rocks
  • Engineering Geology — Interdisciplinary study of geology and engineering
  • Hydrogeology — Study of the nature, occurance, and movement of groundwater
  • Geomorphology — Study of processes that shape the Earth and its landscapes. Geomorphology analyses the dynamics of change in the Earth's landscape and serves as a guide to what the surface of the land looked like in the past and to extrapolate how landscapes might be subject to change in the future.
  • Planetary geology — The intersection of geology and astrophysics; studies the formation of and processes that change rocky planets, moons, asteriods, and other terrestrial celestial bodies.

Rocks and minerals[edit]

Geologically speaking, a rock is a naturally occurring solid aggregate of minerals and mineraloids.[1] To be considered a mineral, a substance needs five traits: one, it is natural, two, it is solid,[note 1] three, it is inorganic, four, it needs a crystal structure, and five, it has a consistent chemical composition.[note 2] Some common examples of minerals include quartz, halite (table salt), and ice. Mineraloids are things that don't necessarily fit all of the criteria, whether due to having no crystal structure (which includes glasses like obsidian and pumice as well as opal), being liquid (like water and petroleum), being organic (like pearls and amber), or having no consistent chemical composition (like coal, which additionally is organic and has no crystals). Some of these definitions have been contested in modern geology. For example, many geological engineers find it easier to ignore the "natural" part of the definition (e.g. by treating concrete as a rock) and may use the term "anthropic rock" to describe materials like these,[2] while many geologists have questioned the "inorganic" part of the definition with the discovery that an increasing amount of supposedly inorganic minerals like magnetite and calcite can be formed by biological processes.[3] Regardless, mineral-stuff and rock-stuff provide important ways of learning about the geological processes and history of the Earth.

Mineral identification[edit]

The minerals that are used to determine Mohs hardness.

There are around 5,400 known minerals, with around thirty to fifty more being discovered every year.[4] Naturally, scientists have developed a few ways in which these minerals can be identified, with the most common indicators being:

  • Color: This isn't often used, as many minerals, including quartz, calcite, and tourmaline, come in many different colors. However, it can be used in some cases. For example, plagioclase feldspar tends to be white or gray, while potassium feldspar has a more pinkish-orange color.
  • Luster: This refers to how a mineral reflects light. Some common lusters include metallic (which looks like the surface of polished metal), pearly (thin sheets reminiscent of pearls), silky (a fibrous texture like silk), vitreous (glassy, like quartz) and earthy (dull).
  • Streak: This is generally more useful than color, as even minerals that vary in color will have the same color powder when dragged across a streak plate. However, this has a couple of disadvantages, as the amount of minerals that this test can be used for depends on the hardness of the streak plate, which is usually at seven hardness (see below) and some relatively safe-to-handle minerals (such as galena) can be dangerous in powdered form.
  • Hardness: In mineralogy, this is a mineral's resistance to scratching; a harder material will always scratch a softer one but never be scratched by anything softer. One approach, the Vickers test, is best for comparing the absolute hardness of minerals. However, the Mohs scale, which uses 10 (relatively) common minerals as reference points (with talc having a value of 1 and diamond having a value of 10), is much more commonly used on the field, as several common objects have well-known Mohs hardnesses (such as fingernails being 2.2, a penny being 3.2, an iron nail being 4, and non-specialized glass being 5.5).
  • Crystal habit: No, this isn't referring to a meth addiction, it's the shape that crystals tend to form in. For example, fluorite forms cubes, quartz forms hexagonal prisms with a pyramid at the tip, asbestos forms fibers, etc.
  • Cleavage: No, not that kind. In geology, cleavage refers to a mineral's ability to split along planes. Minerals with one perfect plane of cleavage (like muscovite) form thin sheets when they break; those with two tend to form prisms (like with hornblende), and ones with three form cubes (rhombohedrons like halite or chlorite). The angles of cleavage can also be used for identification, with feldspar and amphibole both having 2 perfect planes of cleavage, but only feldspar intersects at roughly ninety-degree angles.
  • Density: Different minerals have different densities and are often compared to an equivalent volume of water. For example, gypsum will always be less dense than calcite.
  • Reactivity: As most minerals are made of a specific chemical, one can test how they react with others. Calcite, for example, will noticeably fizz when exposed to hydrochloric acid. As another example, selenite is soluble in water.
  • Taste: Although it's not recommended you do this, some minerals are identifiable by taste. Halite is the most obvious example, as it tastes like, well, salt. Chalcanthite supposedly has a sweet taste — but ingesting it can give you copper poisoning.
  • Magnetism: Only a handful of minerals (most notably magnetite) are ferromagnetic at room temperature, so this can easily identify them.
  • Radioactivity: A Geiger counter will tell you if you are dealing with a radioactive mineral like uraninite.

Igneous rocks[edit]

A pahoehoe lava flow in Hawaii, resulting in the creation of extrusive mafic rocks (i.e. basalt)

Igneous (from ignis, Latin for "fire", compare English ignite) rock is produced by the welling up of molten magma from inside the Earth, which then solidifies as it cools. Geologists divide igneous rocks into "intrusive" and "extrusive". Extrusive rock cools quickly and near the surface via volcanoes and rifts, and tends to have aphanitic, or small, crystals (barely visible with the naked eye), as can be seen in basalt. Obsidian is a special case, as it cools so quickly that crystals don't have time to form and is instead more like glass. Rock is said to be intrusive when magma cools deep under the surface of the Earth. Here the crystals form very slowly and are thus much larger (phaneritic), as for example, the large crystals of quartz, biotite, and feldspar that are visible in plutonic rocks like granite. When the rock starts cooling slowly but later starts cooling at a faster rate, porphyritic rocks are created, which have both large and small crystals.

Igneous rocks can also be divided based on their silica content. Mafic or basaltic rocks comprise primarily low-silica minerals such as amphibole and olivine and make up most oceanic crust. Felsic or rhyolitic rocks are made mainly of high-silica minerals like feldspar and quartz and make up most continental crust. Rocks with large concentrations of both mafic and felsic content are called intermediate or andesitic and mainly form in oceanic-continental subduction zones (as in the region of the Andes). In the deep crust or mantle, sometimes ultramafic rocks like peridotite that often consist of over 90% mafic material are formed; eruptions of ultramafic material used to be common early in the Earth's history, but today are extremely rare. Sometimes gas bubbles will form in lava (similarly to the dissolved CO2 in soda) and this will result in the formation of vesicular rocks like pumice and scoria that have many visible holes in them.[note 3]

Igneous rocks are the only type of rock used for radiometric dating, since melting resets the isotope composition of rock. Igneous rocks can also tell about the history of Earth's magnetic field, as the iron in them will point in the direction of the north magnetic pole; this can give us information such as when the poles switched and can allow us to interpolate the movement of the continents.

Sedimentary rocks[edit]

The Garden of the Gods in Colorado.

Sedimentary rocks are rocks that are formed by the deposition and solidification of sediment, which is matter that settles at the bottom of a liquid (which, in this case, is usually water). They can be split into three general categories that can sometimes overlap, depending on how they were formed.

Clastic sedimentary rocks are aggregates of smaller bits of rocks, minerals, and other solids that have been compacted and fused together by various processes. They may form from remnants of any rock type, including other sedimentary rocks. Over time, the actions of weathering and erosion (via temperature changes, rain, wind, chemical reactions, and the moving action of streams and rivers) break rock down into smaller and smaller pieces. These are washed downstream and eventually deposited in an ocean, river, lake, etc. as sediment. Lithification (turning to stone) of sediments occurs over thousands of years as layers of deposited sediments accumulate, and at depth are compacted by the weight of overlying sediments. The grains are fused together by the dissolution of minerals (just like salt sticking to your body after leaving the ocean) in a process called cementation. As a sedimentary example: quartz sandstones form where quartz sand is the primary sediment, and is typically cemented by silica, though in oceanic settings, calcium carbonate is also quite common. Clastic sedimentary rocks are typically classified according to their grain size (gravel, sand, silt, clay), from conglomerate/breccia,[note 4] sandstone, siltstone, and mudstone/shale in order of decreasing grain size.

Chemical sedimentary rocks are formed when dissolved materials precipitate out of water. For instance, if you mix salt with a glass of water, it will dissolve, but when the water evaporates, the salt will crystalize on the glass. A similar process is how rocks containing minerals like halite and gypsum form in nature. Chemical sedimentary rocks can also form from changes in temperature or acidity in bodies of water; for instance, limestone, a common sedimentary rock consisting of calcium carbonate, is frequently formed when the ocean heats up and can no longer hold as much dissolved calcium carbonate.[5]

Coquina, a form of limestone that is a good example of a biological sedimentary rock. It is formed by broken shells.

Finally, biochemical sedimentary rocks form when organisms die and pile up on each other for many generations and are eventually compressed together. Limestone can also be formed formed by some shell-bearing organisms like corals, bivalves (clams, oysters, etc.), and foraminifera (or forams: a micro-organism). Indeed, the White Cliffs of Dover are composed almost entirely of the skeletal remains of forams. Similarly, chert is a silicic rock that is usually formed from the dissolved skeletons of planktonic creatures, and it also counts as a chemical sedimentary rock. Coal is a sedimentary rock that is created by the buildup of dead plants.

Uniquely, sedimentary rocks are the only rock type that contains fossils, as the melting of igneous rocks or the heat and pressure that metamorphic rocks are subjected to break down any such structures. Sedimentary rocks can also tell us a lot about the environment where they were deposited millions of years ago.

Metamorphic rocks[edit]

This graph shows how shale (a sedimentary rock) changes as it is subjected to increasing heat and pressure. If it were to go any further on this graph, it would melt and become igneous rock.

The original materials of metamorphic rocks used to be either sedimentary or igneous, but have undergone chemical changes due to being subjected to enormous heat and/or pressure. Through the action of the movement of the Earth's crust, particularly continental collision, or when the Earth's crust is pulled downwards towards the interior in a process called subduction (more about this later), rocks of any kind can be subjected to enormous pressure and heat and may transform (or metamorphose) into metamorphic rock. Geologists classify metamorphic rocks by how strongly they have metamorphosed from the original rock. Low-grade metamorphic rocks look a lot like the rocks they came from (e.g. slate is the metamorphosed form of shale, and they look almost identical, but are different physically), while high-grade metamorphic rocks like gneiss look like igneous rocks, but differ in various unusual textural features such as strong veining or lineation. Migmatite is a particularly high-grade metamorphic rock that is subjected to such high heat and pressure that it partially melts; if migmatite were to go through any further metamorphism, it would melt and become igneous rock. Metamorphic rocks can tell us about the processes that have occurred deep underground, although geologists analyze seismic waves to determine structures inside of Earth.

Plate Tectonics[edit]

Tectonic plates of the Earth.
See the main article on this topic: Plate tectonics

Extensive exploration and mapping of the seafloor after World War II led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s Harry Hess of Princeton University and Robert Dietz of the University of California suggested that the seafloor separates along the rifts in mid-oceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967, separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and earthquake activity that take place between the plates. By the end of the 1960s the theory of plate tectonics proved to be a unifying concept that pulled together diverse theories and explained a large body of observations in the field.[6]

About twelve large plates, and several smaller plates, slide over a weak asthenosphere, carrying with them the continental and oceanic lithosphere (comprising the crust and upper mantle). Where continental plates collide, tectonic forces cause the continental crust to compress and buckle upward, producing mountains such as the Himalayas. Oceanic plates are denser than continental plates, and at boundaries called subduction zones, the extra density causes one oceanic plate to sink beneath either a continental plate or less dense oceanic plate, re-entering the mantle and recycling the oceanic crust; parts of the subducting plate melt due to the presence of water and result in volcanic mountain chains like Japan and the Andes. Where plates move apart, at mid-oceanic ridges and at continental rifts like the East African Rift,Wikipedia hot material from the mantle rises to fill the gap, eventually melting to create new sea floor. When two plates slide past each other, it results in the formation of strike-slip faults like the San Andres, which can cause earthquakes due to the rocks fracturing as they slide past each other. In some points in the mantle, hot plumes rise upward, resulting in hot spots like Yellowstone that have volcanism but are generally away from plate boundaries; as the plates move, the hotspot stays in the same place in the mantle, resulting in volcanic chains like the Hawaii-Emperor Seamount Chain that can provide a record of plate motion.

Composition of the Earth[edit]

Volcanic eruption in Hawaii.

Volcanism and deformation bring rocks to the surface of the Earth from depths as great as 50 to 100 km. Scientists can make inferences about some of the properties of the Earth at these depths by studying these rocks. But far more information has been provided through the use of seismic waves created by natural earthquakes,[note 5] and by controlled explosions designed to learn more about the composition of the Earth, including underground nuclear explosions. These data have revealed that the Earth is composed of three main layers: the crust, the mantle and the core. The crust, the outermost layer, varies in thickness from about 5 km under oceans to about 40 km under continents. The mantle consists of an outermost zone about 100 km thick that, along with the crust, is part of the lithosphere, which is relatively strong and makes up the bottom part of tectonic plates. The layer below is a weak, partially fluid solid called the asthenosphere, which ends at a depth of about 300 km. Between 400 km and about 2900 km atoms are packed closer and closer together by extreme pressures, creating a crystalline structure with larger grains, and different minerals, than the layers above. The Earth's core extends from a depth of 2900 km to the center. Seismic wave propagation through the core indicates that the upper two-thirds of the core are liquid, while below a depth of 5100 km pressures are so great that the core becomes solid again.

Age of the Earth[edit]

It is approximately 4.6 billion years old. Young-Earth Creationists, deal with it.


  1. At naturally occurring temperature/pressure combinations
  2. Although some impurities are allowed, which is what explains the various colors of quartz: amethyst has iron impurities, smoky quartz aluminum, rose quartz titanium/manganese, etc.
  3. Pumice is felsic, while scoria is mafic
  4. In conglomerate, the clasts are rounded, while in breccia the clasts' edges are more angular
  5. There are two kinds of seismic waves: s-waves which cause shaking (think the motion of a shaking rope) and p-waves which expand and compress the ground (which are more similar in motion to a slinky). P-waves are faster and can travel between any form of matter, while s-waves only travel through solids, and both of them will travel at different velocities through different densities of matter.


  1. Rock, Robert S. Carmichael, Britannica
  2. Anthropic Rock: a brief history, Richard B. Cathcart, Research Gate March 2011
  3. Magnetite as a prokaryotic biomarker: A review, Concepcion Jimenez-Lopez et al., AGU 22 April 2010
  5. Chemical Sedimentary Rocks, Arizona State University
  6. Press, Frank and Siever, Raymond (1998). Understanding Earth, 2nd ed. W. H. Freeman and Company.